wardjr wrote:AZERATE had a similar issue a week or so ago with a different file so I wonder if he could chime in as to a solution.
Just fixed my problem. I found that my personal issue was that the stl plugin either became corrupt and purged by the system, I accidentally removed the file (highly doubtful), or it grew legs and walked back to Google land. 
I always prefer 8, but have 2013 installed as well....don't ask why. 
Anyway, I had the issue of whenever I attempted to export the stl, once the file directory window popped up, the File Type was blank, thus making it impossible to find the stl since it was saved without a ".xyz" If this is your issue, you will have to reinstall the plugin here: http://extensions.sketchup.com/en/content/sketchup-stl

Before clicking download on the Sketchup site, follow the steps I numbered.
1) Click windows tab in the program.
2) Click preferences.
3) Click extensions.
4) Make sure the stl export plugin box is checked.
Away you go! I have successfully saved and exported a basic shape, and only once since this fix, but it is now working again for the first time in about 2-3 weeks.
Hope this helps 
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